Why can TiAlCrSiYN-based adaptive coatings deliver exceptional performance under extreme frictional conditions?

Abstract

Adaptive TiAlCrSiYN-based coatings show promise under the extreme tribological conditions of dry ultra-high-speed (500–700 m min⁻¹) machining of hardened tool steels. During high speed machining, protective sapphire and mullite-like tribo-films form on the surface of TiAlCrSiYN-based coatings resulting in beneficial heat-redistribution in the cutting zone. XRD and HRTEM data show that the tribo-films act as a thermal barrier creating a strong thermal gradient. The data are consistent with the temperature decreasing from ∼1100–1200 °C at the outer surface to ∼600 °C at the tribo-film/coating interface. The mechanical properties of the multilayer TiAlCrSiYN/TiAlCrN coating were measured by high temperature nanoindentation. It retains relatively high hardness (21 GPa) at 600 °C. The nanomechanical properties of the underlying coating layer provide a stable low wear environment for the tribo-films to form and regenerate so it can sustain high temperatures under operation (600 °C). This combination of characteristics explains the high wear resistance of the multilayer TiAlCrSiYN/TiAlCrN coating under extreme operating conditions. TiAlCrSiYN and TiAlCrN monolayer coatings have a less effective combination of adaptability and mechanical characteristics and therefore lower tool life. The microstructural reasons for different optimum hardness and plasticity between monolayer and multilayer coatings are discussed.